Attorney General Eric Holder says his Justice Department is going after WikiLeaks' Julian Assange. At first glance, it seems like a straightforward case. Almost half of the 250,000 internal State Department cables Assange has published are classified, either at the confidential or the secret level (no top-secret documents are included), and rarely does the target of a criminal investigation commit his alleged crime so publicly.

Holder knows it will not be easy, however. He realizes that as much as we may condemn Assange's actions, they were not obviously criminal. Indeed, just the other day Holder found himself defensively insisting that this is a real investigation, not mere "saber-rattling."

The first challenge, of course, will be to find Assange, who is in hiding both because of this investigation and because he faces rape and sexual molestation charges in Sweden - charges he denies and claims are part of an effort to shut him down. Complicating matters, most extradition treaties do not allow extradition for "political offenses," and Assange is sure to argue that subjecting U.S. foreign policy to public scrutiny is a political act.

But even if the Justice Department finds him, arrests him and extradites him, its work will be far from over. The U.S. government has never successfully prosecuted a media entity for a leak. It is typically much easier to bring such cases against the government officials who do the leaking, because they sign nondisclosure agreements surrendering many of the legal protections they otherwise would enjoy.

Just recently, the department tried to expand the scope of its leak prosecutions to individuals outside the government, in a test case involving two lobbyists for the American Israel Public Affairs Committee, a pro-Israel group. I represented one of the lobbyists, who were indicted for leaking information that had been disclosed to them by high-level U.S. officials during discussions about the Middle East. The men were wiretapped for five years. They were investigated for another year. They were then indicted and spent the next three years skirmishing with the prosecution. And then, on the eve of the trial, the Justice Department dropped the charges.

Why are leak cases so tough?

There is no general law making disclosure of classified information a crime.

What law did Assange violate? It will surprise many that there is no statute making it illegal to reveal classified information. There are statutes that criminalize the disclosure of very specific types of classified information, such as the identity of a covert operative (think Valerie Plame) or "codes, ciphers or cryptographic systems." But there is no catch-all law that simply says, "Thou shalt not disclose classified information."

Indeed, when Congress tried to enact such a statute, President Bill Clinton sensibly vetoed it. His reason: The government suffers from such an overclassification problem - some intelligence agencies classify even newspaper articles - that a law of this sort would end up criminalizing the disclosure of innocuous information. And even that vetoed statute would have applied only to government officials, not to private individuals or journalists.

Instead, prosecutors in the Assange case, like the prosecutors in the AIPAC case I handled, would resort to the Espionage Act of 1917, an archaic, World War I-era statute that prohibits "willfully" disclosing "information relating to the national defense." According to Judge T.S. Ellis in the AIPAC case, this means that the prosecution must prove, among other things, that a defendant knew that the information he was disclosing was potentially damaging to national security and that he was violating the law.

Here, Assange can make the department's case especially difficult. Well before publishing the cables, he wrote a letter to the U.S. government, delivered to our ambassador in London, inviting suggestions for redactions. The State Department refused. Assange then wrote another letter to State, reiterating that "WikiLeaks has absolutely no desire to put individual persons at significant risk of harm, nor do we wish to harm the national security of the United States."

In that second letter, Assange stated that the department's refusal to discuss redactions "leads me to conclude that the supposed risks are entirely fanciful." He then indicated that WikiLeaks was undertaking redactions on its own.

The First Amendment still matters.

The First Amendment, of course, protects both freedom of the press (yes, WikiLeaks is the press) and freedom of expression. That is one reason Holder is not investigating this newspaper or the New York Times, even though both are publishing extensive details from the cables: It is the Justice Department's practice to refrain from bringing leak indictments against traditional media outlets.

Holder may feel emboldened to move against WikiLeaks because it does not have the look or feel of traditional news media. Still, Assange can rely on the courts to be vigilant in protecting his First Amendment rights and affording him the same protection that traditional media enjoy.

The fact that classified information is involved does not preclude First Amendment safeguards. In the AIPAC case, Judge Ellis rejected the prosecutors' categorical - and dangerous - argument that when classified information is at issue, the First Amendment affords no protection. Of course, the First Amendment is no license to disclose the recipe for the plutonium bomb to Osama bin Laden. But the Justice Department would have to prove that Assange's disclosures were so dangerous to national security as to override the First Amendment. In the words of Justice Oliver Wendell Holmes Jr., the prosecution would have to demonstrate that what the defendant did was as immediate and as dangerous as "falsely shouting fire in a theater." That is a heavy burden to meet.

More secrets would have to be disclosed at trial.

It is very difficult to prosecute a leak without disclosing additional classified information in the process. After all, how does the government convince a jury that releasing a cable's contents damaged national security? By calling an expert witness who tells the jury what information we were getting from a specific foreign government before the disclosure and what happened after the disclosure. Imagine the government spending $1,000 to investigate internal waste of $100 - it might prevent future waste, but the cost-benefit analysis is far from obvious.

The damage is hard to assess.

In some instances, secrecy is clearly vital and disclosure harmful. That is particularly true in the military sphere, but it holds for foreign policy as well. Indeed, the State cables themselves talk about how previous publicity caused the Pakistanis to drag their feet on a plan to remove highly enriched uranium from an aging reactor. But in a prosecution of Assange, his defense would argue vigorously that prior assessments of harm due to leaks have proven over time to be wrong.

In 1971, Solicitor General Erwin Griswold asked the Supreme Court to bar publication of the Pentagon Papers because it would cause a "grave and immediate danger to the security of the United States." Twenty-eight years later, he reversed his position in an op-ed piece in this paper. "I have never seen any trace of a threat to the national security from the publication" of the Pentagon Papers, Griswold wrote. Moreover, he expressed the view that "there is very rarely any real risk to current national security from the publication of facts relating to transactions in the past, even the fairly recent past."

What took 28 years to happen with the Pentagon Papers is already happening with the WikiLeaks cables. Although the State Department is of the opinion that Assange's leaks have done serious damage to our national security - Secretary of State Hillary Rodham Clinton has called them "an attack on America" responsible for "endangering innocent people" and "sabotaging the peaceful relations between nations" - Defense Secretary Robert M. Gates, a savvy Washington veteran, has expressed a different view.

"I've heard the impact of these releases on our foreign policy described as a meltdown, as a game-changer and so on," Gates told reporters at the Pentagon last week. "I think those descriptions are fairly significantly overwrought. . . . Is this embarrassing? Yes. Is it awkward? Yes. Consequences for U.S. foreign policy? I think fairly modest."

If Assange is ultimately charged with disclosing information that is potentially damaging to national security, Eric Holder now knows who Assange will call as his first witness: the secretary of defense.

Baruch Weiss, a litigation partner at Arnold & Porter, specializes in white-collar and national security matters. He is a former federal prosecutor and served in the Treasury and Homeland Security departments.